Not doubting your issues chuck - just reassuring you that it’s doable on your hardware, so doubtful switching from Mac to Mac will make any real difference, and that the issue must be software-based somewhere along the line.
Am not sure of difference regarding Syphon sending video vs generated graphics - a frame from the frame buffer is a frame from the frame buffer (?) - beyond my knowledge there! Airbeam is firing over 4x streams of H264-encoded video from iPhone cameras live, and I also tend to have a HDMI Camera hooked up via Black Magic Intensity Shuttle, but don’t really ferry it’s signal around by Syphon.
I _do_ know there are differences in the Syphon output of different apps. e.g.: If I use native Syphon receivers I occasionally have issues receiving streams from processing (just show up all white), and end up resorting to the FFGL Syphon Receiver plugin from http://syphon.v002.info - could be worth a try to see if there’s a difference for you between running the Isadora native Syphon receiver and FFGL Syphon receiver... both co-exist happily in Isadora and can be used simultaneously in a patch for comparisons.
MagicMusicVisuals = https://magicmusicvisuals.com, which I mainly use for running custom GLSL shaders into Isadora patches, with the shaders’ animated properties triggered via frequency-gated multi-audio input channels coming from MOTU 828MkII & 8PRE (firewire) & Leap/Kinect motion data via OSC.